The US defence secretary, Donald Rumsfeld, yesterday signalled a transformation in the US military presence in the Gulf region by announcing that all but a handful of American troops will be pulled out of Saudi Arabia by summer's end.

Despite vociferously insisting this week that the US is not "pulling out" of the country, the defence secretary's announcement amounted to that, reducing the 5,000 troops there to 400, who will mainly be there to train Saudi soldiers.

The Prince Sultan air base, largely rebuilt at a great cost to the US, will be largely abandoned, with none of the 200 American planes currently there remaining by the end of August.

Mr Rumsfeld, in a joint press conference at the air base with Prince Sultan, the Saudi defence minister, insisted the decision was a "mutual agreement" motivated by reasons of military strategy.

Now that the Operation Southern Watch no-fly zone no longer exists, the Pentagon argues in public, there is no need for planes to be based in Saudi Arabia to patrol it.

Furthermore, Riyadh refused to let American planes take off on ground-attack strikes from its airfields during the Iraq war, because it would look as if Saudi Arabia was backing an attack on fellow Muslims - a restriction that irritated US military commanders.

Non-strike missions - for refuelling, reconnaissance and other purposes - were permitted.

The announcement "does not mean we requested them to leave Saudi Arabia, but as long as their operation is over, they will leave," Prince Sultan said.

But behind the military considerations, the move is a major positive gesture by Washington towards the Saudi royal family, for whom a high-profile US military presence creates internal pressure, stoking militant opposition.

All but four of the alleged September 11 hijackers were Saudi, and the US troops there have been a key propaganda point made by Osama bin Laden and al-Qaida.

Hints that a major restructuring of the American military footprint in the region were under way came earlier this week, with the news that the US was planning to move the command centre of its air operations to the newly re-equipped al-Udeid air base in Qatar.

Officials confirmed yesterday that the switch had been made.

Before the war, the US had moved large amounts of technology and equipment to al-Udeid in anticipation of opposition from Riyadh.

"Nothing's going to be torn down," Rear Admiral Dave Nichols, deputy air commander for Central Command, said of the Prince Sultan base. "It'll remain wired, but most of the computers and whatnot will be taken out."

"There are political advantages for both," said Tim Garden, security analyst at the Royal Institute of International Affairs.

"The US will have greater freedom of action, the Saudis will feel more comfortable, and neither of them will have to mention that it was a key demand of Osama bin Laden."